After three horribly failed attempts with the cannon at the Darkmoon Faire, I discovered what appears to be a conflict with the MyRolePlay addon.

With both nUI and MRP installed, mousing over the area where nUI displays buffs causes the MRP tooltip describing my character to pop up, just as if I'd moused over my character portrait in the default Blizzard UI, instead of the tooltip for the buff being moused over.

As a result, I was unable to right-click the cannon's "Slow Fall" buff off (and I assume the same would apply to any other buff I tried to right-click off of my character).

Isn't it easier to use the action bar for that, or just press 1 (with the default keybindings)?

Yeah, I finally noticed the action bar change on my 4th attempt. I missed it before because I'm just accustomed, from the previous version of the cannon, to right-clicking the buff icon to cancel it.

Nevertheless, there are times when you want to right-click to remove a buff (Hallow's End costumes, Arcane Brilliance when a "helpful" mage puts it on your warrior or rogue, etc.) that don't spawn an action bar button, and the MRP character tooltip covering the nUI buff icons gets in the way of that.

To further elaborate, it's not the position of the MRP tooltip that is actually the problem. The "conflict" is that the tooltip is being activated by mousing over the buff icons, and so while I'm mousing over one thing, the game thinks I'm mousing over something else, with the result that I think I'm clicking on one thing, and the game thinks I'm clicking on something else, and so my click does not have the desired result.

The mouse pointer doesn't show up in the screenshot (I thought there was a setting for that, but I can't find it now), but it's hovering right over the "Honor Among Thieves" buff icon. The tooltip isn't actually covering the icon, but the game just doesn't realize that it's the buff icon I'm hovering over; it seems to think I'm hovering over my character portrait, and behaves accordingly.

Side note: I understand that the actual problem here might just as easily be something on MRP's end, but I'm not a coder myself and have no way to diagnose that. I'll drop MRP's author a note to point them to this thread.

…Arcane Brilliance when a "helpful" mage puts it on your warrior or rogue, etc.…

Don't blame the mage. Here's the tooltip for the spell:

Infuses all party and raid members with brilliance, increasing their maximum mana by 2126 and spell power by 6% for 1 hour. If target is in your party or raid, all party and raid members will be affected.

Besides, why bother turning it off? If you're a warrior or rogue it doesn't affect you at all.